Steinberger added, "We don't like letting you down. Our teams are working around the clock to resolve these issues so that you can have the experience you've come to expect."

He said the promotion would be "pausing for the time being."

Comics readers were told to post their e-mail addresses on a form to be notified when the offer would be back up and running.

Marvel posted a statement to its website: "Marvel fans assembled in record numbers to take advantage of the landmark Marvel #1 digital comics sale! The demand has been so overwhelmingly positive, in fact, that a number of technical issues have arisen with our technology partner in getting the more than 700 free issues to each of you. When we have further updates from our partner, we will share them."

The advent of reading comics digitally, especially on tablets, has been seen by some as the future of the comics industry, as print readership continues to decline.

To that end, Marvel has increased its presence at SXSW, this year announcing products such as the weekly Infinite Comics, which are specifically designed with the digital reader in mind, and Project Gamma, which adds audio to the digital reading experience.